<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script src="https://unpkg.com/amfe-flexible@2.2.1/index.js"></script>
</head>
<style>
/* rem相对值,相对于html标签的font-size
    默认html的字体16px
 */
/* html {
    font-size: 20px;
} */
#box{
    /* width: 300px; */
    width: 10rem;
    height: 200px;
    background-color: pink;
}
</style>
<body>
    <div id="box"></div>
</body>
</html>